Fracture toughness anisotropy by indentation and SEVNB on tetragonal PZT polycrystals

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Abstract

The fracture toughness of several poled and unpoled well characterized PZT materials was studied by indentation and bending techniques to determine the materials intrinsic fracture toughness anisotropy. Indenter loads were applied both in perpendicular and parallel directions to the polarization axis. Hardness, Hv, Hk, Young's Modulus, E, and fracture toughness, K1c, measured using the indentation technique showed anisotropy in poled samples. The observed anisotropy was partly related with reorientation of ferroelectric domains. Values of K1c were determined using the single edge V-notch beam (SEVNB) technique to determine the influence of experimental artifacts associated with the indentation method. A significantly lower anisotropy in fracture toughness was observed.

Introduction

PZT ([PbZr]TiO3) is a ferroelectric ceramic at room temperature. At the Curie point the material transforms from cubic (paraelectric) to tetragonal, and the strain generated is relieved by the formation of twin related domains. These twins, each one a single ferroelectric domain, can be switched (reoriented) by an electric field or a mechanical stress [1]. PZT is the most common functional material in piezoelectric devices for load transmission, e.g. transducers and actuators. Fracture toughness anisotropy in ferroelectric materials was revealed by Okazaki [2]. At the time most ferroelectric applications did not require the use of high operating stresses. The need for continuously operated, large displacement actuators has caused an increasing interest in the study of the degradation of the electromechanical properties of PZT more recently, specifically on the damage produced by mechanical fatigue [3], and the behavior of cracks under both mechanical loads [4], [5] and electric fields [6]. Domain switching (twinning) in the near stress field of a crack tip is considered a possible toughening mechanism [7], [8], although the subject remains to be clarified. The apparent fracture toughness measured using the indentation method is greatly influenced by the crack direction relative to the poling direction [2]. However the comparison of fracture toughness data obtained from different crack propagation planes in poled specimens has been contradictory [7], [8], [9]. The real effect on crack evolution of electrically or stress induced ferroelectric domain reorientation at the crack tip is yet to be clarified. The objective of this study is to investigate the fracture toughness anisotropy in PZT by different methods to determine if it is caused solely by crack growth induced domain switching during crack growth or is also associated with the experimental technique used, as was suggested by the authors in a previous work [9].

Section snippets

Experimental procedure

The materials were a commercial PZT of common industrial use, named P, and two PZT materials with very different electromechanical properties: a soft PZT, called SP, with high piezoelectric constants and low mechanical quality factor QM, and a hard PZT, called HP, with low piezoelectric constants and high QM.

Knoop indentations were made to obtain E/Hk from the ratio of the length of diagonals, using the method described in [10] Vickers indentations, with applied loads ranging from 2.5 to 10 kg,

Results

The PZT materials showed some anisotropy with respect to the poling direction was founded, in the surface parallel to the poling direction the values of hardness calculated from the diagonal length of the indentation impressions were always slightly lower than in the unpoled sample and in the surface normal to the poling direction the hardness took higher values.

The fracture toughness, K1c, were determined using the indentation and the single edge notch beam techniques. Values of K1c for the

Discussion

The c1 and c2 indentation cracks define the toughest and the least tough directions, respectively, in all the materials. The crack growth in the geometry corresponding to c3 cracks in poled samples is of interest for the analysis given the complexity of the measurement of indented fracture toughness in anisotropic materials [12]. The same indented crack growth behavior has been observed before by the authors [13]. We considered that this large fracture toughness anisotropy may arise partly from

Conclusions

In the present work fracture toughness was measured in a number of unpoled and poled PZT materials, using the indented surface cracks and the four-point bending SEVNB techniques, to determine the effect of poling of the fracture toughness and the origin of the anisotropy previously observed by the indentation method. The results seem to confirm the overestimation of toughness anisotropy measured by the indentation technique with respect to the more reliable SEVNB technique. A small toughening

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